Improved vinyl acetate homo- and copolymer-based emulsion adhesives characterized by superior "easy-clean" properties are prepared by the addition thereto of specific water soluble, ionic salts. Furthermore, when such salts are added to those emulsions adhesives which are suitable for remoistening applications, the resultant products are also characterized by improved "lay-flat" performance.
Vinyl acetate-based emulsion adhesives are used for a wide range of applications in industry. Typically, they are used in remoistening, stenciling, laminating, packaging and other high speed adhesive applications. It will be recognized that the high speed machining of these adhesives results in the formation of deposits of dried adhesive on the various machine parts which eventually interferes with the machining. Therefore, the need to clean off these deposits quickly and simply with water (a property referred to as "easy-clean") and thereby minimize shut-down time is being emphasized by adhesive users. Heretofore, the "easy-clean" properties have been achieved, for example, by the use of relatively large amounts of polyvinyl alcohol. However, polyvinyl alcohol is expensive and, in some cases, the use thereof may undesirably thicken the adhesive or may react unfavorably with the substrate to be adhered as in the case of "patch-gumming" of polystyrene panels in the manufacture of envelopes.
In a related area, certain vinyl acetate-based emulsion adhesives also require a degree of water-sensitivity in order to provide for remoistenable applications. However, the use of these aqueous emulsions to produce remoistenable adhesives results in the swelling of the paper fibers and eventual curling of the substrate, a problem which is further aggravated upon drying. Attempts have been made to avoid or minimize the curling by the addition of various humectants or plasticizers, however, none, to date, have resulted in the production of a satisfactory "lay-flat" aqueous adhesive. The latter property has been achieved only by passing the curled substrate over knife edges or rollers in order to destroy the continuity of the adhesive film and thereby allow the substrate to resume its flat configuration.
It is therefore an object of the present invention to provide vinyl acetate-based aqueous emulsion adhesives which are characterized by improved easy-clean properties. It is a further object of the invention to provide specific adhesives, which, in addition to possessing easy-clean properties, are also useful as remoistenable adhesives which maintain their lay-flat configuration even after drying.
These and other objects will be apparent from the following description of the invention.